Composite electroless coating is a new generation of composites which can be derived via electroless plating techniques. The following patents and article reflect upon the state of the art, the techniques which are used, as well as those particulate matters which may be incorporated within the electroless plating matrices: U.S. Pat. Nos. 3,617,363; 3,674,447; 3,753,667; Reissue No. 29,285; R. Barras et al, "Electroless Nickel Coatings-Diamond Containing", Electroless Nickel Conference, Cincinnati, Ohio, November, 1979; and British Pat. No. 1,476,024. These Patents are included herein by reference.
Though electroless plating may be applied to a wide variety of substrates, the coating of metallic substrates is of great technological interest for achieving any of several properties on the initial substrates (e.g., corrosion protection, wear-resistance, etc.). However, plating may be carried forth on non-conductor and semiconductor type substrates as well. Though the mechanism of composite electroless plating is not fully understood, it is believed that the insoluble particulate matter suspended within the electroless plating composition is entrapped during the electroless plating process build-up. For an effective entrapment, the insoluble particle must attach itself to the surface and permit the conventional electroless plating process to proceed without disturbance, and encapsulate the particle(s) without interruption of the plating process.
It is therefore recognized, since the particulate matter does not appear to participate in the actual (basic) mechanism (see (1) R. M. Lukes, Plating, 51, 969 (1964); (2) N. Feldstein et al, J. Electrochem. Soc., 118, 869 (1971); (3) G. Salvago et al, Plating, 59, 665 (1972)) of the conventional electroless plating but rather is entrapped, that it is therefore essential that there be a high probability for the particulate matter to "stick" to the surface and result in fruitful entrapment rather than contacting the surface and falling off the surface into the bulk solution. It is also recognized that the electroless nickel matrix provides "cement" for the entrapment of the particulate matter. Moreover, it is undesirable for the particles to become autocatalytic.
In general, in the present invention, particles in the size range of 0.5 to 75 microns may be contemplated. It is further preferable to select the desired particle size with a narrow particle size distribution. In most applications, generally speaking, the particle size is in the range of 15 to 30% by volume, though it is possible, particularly with higher temperature and/or high bath load concentration, to achieve particle loading within the deposit exceeding 40% by volume.
In the case of diamond particulate matter, especially diamond of a polycrystalline nature manufactured by an explosion process, preferred particles may be selected in the range of 1 to 9 microns in size.
Table 1 provides the hardness of selected materials. The materials demonstrated in Table 1 are diamond, silicon carbide, corundum, tungsten carbide, nitrided steel, hard chrome, etc. It is therefore realized that for wear-resistant application it would be most useful to incorporate particulate matter having a greater hardness value in comparison to the metallic matrix derived via electroless plating or those materials or particles which are higher than the nickel phosphorous alloy which reaches a hardness of approximately 69 Rockwell C units with heat treatment, as is well known in the art.
TABLE 1 ______________________________________ Hardness of Selected Materials Material Hardness (Vickers-Kg/mm.sup.2) ______________________________________ Diamond 10,000 Silicon Carbide 4,500 Corundum (Al.sub.2 O.sub.3) 2,400 Tungsten Carbide 1,300 Nitrided Steel 1,110 Hard Chrome Plate 1,000 (R.sub.c 70) Nickel Phosphorous Alloy 950 (R.sub.c 69) Hardened Steel 900 (R.sub.c 62) P-2 Steel 400 (R.sub.c 38) ______________________________________